Boiler plants, particularly those having coal-fired boilers, are well-known sources of pollution. Sulfur emissions from these plants has become a matter of heightened concern in recent years, in light of the perceived effects of "acid rain." Although pollutant emission reduction has long been the object of scientific inquiry, there is still a great demand for methods and apparatus that are effective, easily retrofit onto existing plants, and which convert the collected pollutants into useful and valuable byproducts.
The prior art recognizes that cooling flue gases that contain sulfur trioxide and water vapor will result in condensation of sulfuric acid. See U.S. Pat. No. 4,526,112 to Warner; U.S. Pat. No. 4,874,585 to Johnson, et al.; U.S. Pat. No. 4,910,011 to Dorr, et al.; and Bovier, Sulfur-Smoke Removal System, 26 Proc. Am. Power Conf. 138 (1964). Other systems of the prior art employ high-volume sprays of water or other liquid agents to scrub particulate matter and pollutants from the flue gas. See U.S. Pat. No. 3,442,232 to White; U.S. Pat. No. 3,456,928 to Selway; and U.S. Pat. No. 3,770,385 to Grey et al.
It was further recognized that when a tube-type heat exchanger is located in the path of a flue gas flow, the particulate material in the gas can collect on the heat exchanger tubes. Cleansing sprays have been provided to accompany the heat exchanger according to some prior art methods to wash the collected particulate from the heat exchangers. See U.S. Pat. No. 4,452,620 to Dosmond; and U.S. Pat. No. 4,526,112 to Warner.
Although the aforementioned systems are more or less effective in removing some degree of the pollutants from the flue gas, they merely shift the environmental burden from atmospheric waste to surface waste. The prior art has recognized that strictly gas phase reactions in flue gas can produce ammonium sulfates or sulfites. See C. C. Shale, Pollution Control and Energy Needs. However, the prior art failed to grasp that these products could be collected and sold as fertilizer byproducts and that noxious pollutants can be removed from power plant flue gas by such reactions.
Attempts have therefore been made to convert pollutants into useful end products as they are removed from flue gases. My prior patents teach how to combine an ammoniacal substance with sulfur oxides in the flue gases to form ammonium sulfites or ammonium sulfates, either in the form of crystals (U.S. Pat. Nos. 5,198,201 and 5,230,870) or in a condensate solution (U.S. Pat. Nos. 5,122,352 and 5,273,727). Since ammonium sulfate is soluble in its own weight of water, this byproduct can be collected in the condensate from the cooled flue gas. However, ammonium sulfate forms crystals which can deposit upon heat exchanger tubes, so that the apparatus must be periodically cleansed with additional water. Thus, there is a need for improvement in the operation of such processes with regard to the collection and recovery of ammonium sulfate or related compounds for reuse as fertilizers or the like.